Antenna feedhorn support structure



Dec. 31, 1968 A. COHEN ETAL 3,419,871

ANTENNA FEEDHORN SUPPORT STRUCTURE Filed Oct. 25, 1965 Sheet of 2 ALBERT .COHEN' CHARLES w. CREASER,Jr

Dec. 31, 1968 EN 'ET AL 3,419,871

ANTENNA FEEDHORN SUPPORT STRUCTURE Filed Oct. 23, 1965 Sheet 2 6r 2 q: U) N E O 2 a]. 5

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g g CHARLES'W. cR AsER,Jr ZY OI I -//VVENTOR$ T 901 0| NOLLDBS ssoaa avcva ATTORNEYS United States Patent ANTENNA FEEDHORN SUPPORT STRUCTURE Albert Cohen, Stow, Mass., and Charles W. Creaser, Jr., Amherst, N.H., assignors to Communication Structures Incorporated, West Concord, Mass., a corporation of Massachusetts Filed Oct. 23, 1965, Ser. No. 503,749 8 Claims. (Cl. 343-781) ABSTRACT OF THE DISCLOSURE In an RF antenna assembly having an RF feed supported by spars from a reflector, the spars are made of ogival cross section in which two arcs symmetrical about a common chord intersect each other at acute angles to form the edges of each spar, and the spars are each disposed with its edges facing respectively toward and away from the source of RF radiation.

This invention relates to RF antennas and particularly to antennas using circular reflectors with an RF feed device supported facing the reflector.

In the fields of radar, microwave communications and space research, there has been constant striving and progressive improvement in the efliciency of electromagnetic wave transmitting and receiving systems. The antenna has been found to be an extremely sensitive part of these systems and has been improved steadily ever striving to gain fractional decibels of efliciency.

RF reflectors have been made with surfaces to extreme tolerances in a class with high quality optical reflectors for optimum benefit. Feedhorns have been designed and redesigned at research costs in the hundreds of thousands of dollars. Structural frameworks supporting feedhorns have been made with open girder construction, I beams, and cylindrical posts of materials ranging from metal to various plastics with the aim of providing rigid structural support with minimum blocking of the aperture.

The effect of some of these factors on antenna efliciency is illustrated by way of example in the following table taken the JPL Space Programs Summary, Fall 1963, No. 37-24, vol. III, p. 25.

Item Associated Aperture gain, db elficiency Theoretical maximum 56. 24 1. 000 Illumination factor (includes phase loss 1. 06 Gain for perfect surface and no quadripod +55. 18 0. 783 Surface tolerance loss (0.032-in. rms.) 0. Gain for no quadri 0d +55. 13 0. 775 Gain loss for 100 0 opaque quadripod (machine computed) -1. 19 Gain loss for 63% opaque quadr1pod. 0. 73 Predicted gain for 63% opaque quadripod +54. 40 0. 655 Measured gain (p.e.) 54. 40=|=0. 0. 655=t=0. 020

It can be seen in this table that the measured gain is down only 1.84 db from the theoretical maximum so that any improvement will only be in fractional dbs. Of the losses presented, the significant ones are illumination factor and quadripod. The illumination factor is associated with the feedhorn and the quadripod is of course the structure supporting the feedhorn. To improve on feedhorns at the present stage of the art can be expected to run into rather astronomical research dollars, which leaves the supporting structure for the feedhorns as a place of interest for improvement.

In this regard it is of interest to note that the art generally describes the detrimental characteristics of feedhorn supports in terms of aperture blocking. This has perhaps led previous researchers a little astray. The prob- Patented Dec. 31, 1968 lem is related to more complex attributes of electromagnetic wave action. The term backscattering has been applied which perhaps still expresses an oversimplification.

A little thought along these lines soon suggests that one can block the aperture to a very significant degree with most any material without significant loss as long as the electromagnetic wave fronts are not seriously disrupted.

Now in accordance with the present invention we have found that spars with ogival cross-sections provide a substantial reduction in backscatter as compared to other shapes used to support RF feedhorns. The term ogival here is used to describe a geometric figure formed by an are drawn symmetrically on opposite sides of its chord. This looks like an oval with pointed ends. Also in accordance with the invention we have found that such spars can be readily extruded in two mating pieces. Assembly from two pieces permits the ready introduction of a novel viscoelastic dampening arrangement for reducing vibration and likewise permits use of the spar interior for carrying a transmission line to the feedhorn. Thus it is an object of the invention to define novel supporting spars for positioning an RF feedhorn in front of a reflector.

It is a further object of the invention to define an extruded component of a supporting spar.

It is a further object of the invention to define vibrationally damped supporting spars for RF feedhorns having ogival cross-sections.

Further objects and features of the present invention will become apparent upon reading the following specification together with the drawings in which:

FIG. 1 is a projection of an antenna assembly in accordance with the invention.

FIG. 2 is a projection of a feedhorn support spar in accordance with the invention.

FIG. 3 is a cross-section of an extruded component of a spar such as in FIG. 2.

FIG. 4 is a graph illustrating backscattering for different geometric shapes.

FIG. 5 is a graph showing backscatter from a 6 inch by 3 inch cross-section ogival spar as it is rotated about its longitudinal axis.

FIG. 6 is a graph showing backscatter from a 6 inch diameter cylindrical spar rotated about its longitudinal axis.

FIG. 7 is a graph showing backscatter from an 8 inch by 4 inch cross-section ogival spar rotated about its longitudinal axis.

FIG. 8 is a graph showing backscatter from an 8 inch diameter cylindrical spar rotated about its longitudinal axis.

An air search radar antenna is depicted in FIG. 1 with a spherical reflector 10, RF feed device 11, and a quadripod mounting consisting of support spars 12, 13, 15, and 16. The support spars are mounted at one end on the rim 17 of reflector 10. The other end of each support spar is fastened to mounting means 18 associated with RF feed device 11. The exact configuration of either the reflector or RF feed device is not pertinent to the present invention.

The support spars are made with ogival cross-sections and are each positioned so that one sharp edge or nose faces reflector 10 and the opposite nose faces away from reflector 10.

FIG. 2 illustrates a hollow metal support spar 12 with a diameter d, width w and nose angle The cross-section of the spar is ogival with two opposite sharp edges. Electromagnetic waves upon striking one of the sharp edges apparently flow around the surface of the spar and meet again at the opposite edge. The amount of backscattering La d A 2(1cos The preferred cross-sectional shape will conform with where:

k=wavelength of the RF energy La/ \=arc length of ogive =nose angle (in radians) d diameter (curve center to curve center) n=integer The arc length will be determined with different mathematics when the arc is not a circle arc, but as long as the arcs are fairly close to circle arcs, backscattering can be reduced.

As can be seen in FIG. 1, spars 12, 13, and 16 are not normal to the propagation path. Thus in computing preferred dimensions the ogive arc lengths must be taken as surface arcs aligned with the propagation path of the antenna.

The nose angle gb is preferably kept as small as is consistent with good design. Backscatter increases with increasing nose angle. While no critical maximum nose angle has been determined, it has been found practical to keep it no greater than 75.

The actual gross dimensions in any given antenna system will depend on the structural strength required. The support spar 12 of FIG. 2 is made by interlocking two extruded members together.

One such extruded member 20 is depicted by crosssection in FIG. 3. Member 20 is readily extruded from aluminum with tongue 21 and channel 22 formed in the extruding process. Two members identical to member 20 can be interlocked by sliding tongue 21 of one into channel 22 of the other and vice versa to make spar 12.

In addition to the low cost of manufacture when an extrusion process is used, the open members provide a readily accessible interior for further improvements.

Thus, in FIG. 3 solid metal rod 24 and metal bar are disposed along the interior length of spar 12. Metal rod 23 is secured to member 20 by viscoelastic material 24 and bar 25 is secured to member 20 by viscoelastic material 26. Bars 27 and 28 are similarly secured inside this bar by viscoelastic material. These rods and bars will not follow resonant vibrations in the aluminum shell walls of the spar. Thus any deflection in the aluminum shell results in a shear stress in the viscoelastic material dampening out resonant conditions.

Where radar antennas or the like are subjected to high Winds or other conditions that tend to promote resonant vibrations, dampening as described above extends lifetime and reliability. This also improves stability in the feedhorn position with a resultant improvement in beam pattern. Further details of dampening means for use in the present invention will be found in the Journal of Engineering for Industry, November 1961, pp. 403 to 424, in an article entitled Dampening Structural Resonances by Jerome E. Ruzicka.

W. E. Blore examined radar backscattering from a number of circular solids. His results are described in the IEEE Transactions on Antennas and Propagations, vol. AP-12, No. 5, September 1964, pp. 582 to 590. His results are interesting since they are closely analogous with the figures obtained by experimentation in accordance with the present invention.

The present invention deals with extended spars having cross-sections similar to the cross-sections in Blores circular solids. Some of Blores results are depicted graphically in FIG. 4. The curves in this figure shows backscattering from a circular ogive 31, a sphere 32, a cone 33, double-backed cone 34, cone sphere 35, and doublerounded cone 26. Our experimentation with a cylinder and a spar of ogive cross-section has shown the same relative difference in backscattering as is indicated by curve 31 for a circular ogive and curve 32 for a spar. It is worthwhile to note that the circular ogive at its worst still shows a marked improvement over the sphere.

FIG. 5 is a graph showing a curve 40 of backscattering measurement from an ogive 6 inches by 3 inches in crosssection. These dimensions are the w and d dimensions illustrated in FIG. 2. Measurement was made as the ogival spar was rotated about its longitudinal axis. It Will be seen that backscatter goes from a peak of about 15 db to a minimum approaching 40 db as the spar is rotating from broadside to nose-on facing the source of radiation.

FIG. 6 shows a graph having a curve 41 representative of backscatter from a 6 inch diameter cylinder varying between about 16 db and 26 db. This curve should be a straight line but varied due to a slight off-axis rotation of the cylinder in the experiment.

FIG. 7 illustrates a graph in which a curve 42 depicts backscatter which is down close to 40 db with an 8 inch by 4 inch ogive positioned edgewise facing the radiation source.

In FIG. 8 a graph is illustrated with a curve 43 depicting backscatter from an 8 inch diameter cylinder. The minimum backscatter as the 8 inch cylinder was rotated is shown to be about -22 db.

The particular diameters of ogival spars and cylinders used in the above tests were selected as providing the most closely analogous support strength when used as antenna feedhorn supports.

While the invention has been described in relation to specific embodiments, various modifications thereof will be apparent to those skilled. in the art. For example when the spars are not designed critically to a specific frequency, it is not necessary that the longitudinal surfaces be parallel and the spars may be tapered to a maximum diameter at one end (i.e. the feedhorn end) or to a maximum diameter in the middle. Thus it is intended to cover the invention broadly within the spirit and scope of the appended claims.

What is claimed is:

l. A support spar for an RF feed device in an RF antenna assembly comprising an elongated member having a cross section defined by two identical arcs symmetrically disposed about the chord of one of said arcs and forming substantially sharp edges Where the arc and the chord intersect, the arcs forming an acute angle at each edge, in which said member is comprised of two halves with interlocking edges, each half including one of said angles and a portion of each of said arcs forming said one angle.

2. A support spar according to claim 1 in which said two extruded halves are identical halves and said interlocking edges comprise a tongue extruded along a first longitudinal edge of each half and a channel extruded along a second longitudinal edge of each half.

3. A support spar for an RF feed device in an RF antenna assembly comprising an elongated member having a cross section defined by two identical arcs symmetrically disposed about the chord of one of said arcs and forming substantially sharp edges where the arc and the chord intersect, the arcs forming an acute angle at each edge, in which vibrational damping is supplied by at least one solid metal member extending in the elongated dimension of said spar and adherent to an inner wall of said spar through a viscoelastic medium whereby deflection in said wall sets up a shear force in said medium due to relative motion between said wall and said metal member.

4. An RF antenna assembly comprising a reflector, an RF feed and support structure for suspending said feed facing said reflector comprising a plurality of support legs of ogival cross-section each comprising an elongated member having a cross-section defined by two identical arcs symmetrically disposed about the chord of one of said arcs and forming substantially sharp edges where the arc and the chord intersect, the arcs forming an acute angle at each edge, each fastened at one end adjacent to said reflector and at a second end adjacent to said feed and disposed with the sharp edges of the ogive facing respectively toward and away from the source of radiation.

5. An RF antenna assembly according to claim 4 in which said reflector is a spherical reflector and said one end is in each instance fastened to a point at the rim of said reflector.

6. An RF antenna assembly according to claim 4 adapted to operate at a predetermined frequency in which said support legs have surfaces alOng said arcs in a direction parallel to the propagation path equal to a number of half wavelengths of said determined frequency.

7.- An RF antenna assembly according to claim 4 in which each said support legs has an internal solid metal member extending longitudinally and adherent to an in ternal wall by means of viscoelastic material.

8. A member for assembly of elongated spars of ogival cross section having arcuate walls intersecting a common chord along two lines, comprising a member having a cross section defined by sections of two identical arcs intersecting at an acute angle in a line and passing from said line half the length of said chord to free longitudinal edges, a first longitudinal edge formed as a tongue and a second longitudinal edge formed as a channel whereby two of said members can be interlocked by engaging the tongue of one into the channel of the other and vice versa, a solid metal rod being secured within said member by viscoelastic material at the apex of said angle along the elongated dimension of said member, for setting up a shear force in said viscoelastic material due to relative motion between said member and said rod.

References Cited UNITED STATES PATENTS 999,267 8/ 1911 Slick 52-731 1,939,558 12/1933 Loudy 52-731 2,626,353 1/1953 McGee 343887 2,714,161 7/1955 Featherstun 343904 2,940,078 6/1960 Bodmer et al. 343840 3,010,106 11/1961 Lippitt et al. 343 915 3,111,203 1l/1963 De Ridder 343--915 FOREIGN PATENTS 903,469 8/ 1962 Great Britain.

ELI LIEBERMAN, Primary Examiner.

US. Cl. X.R. 343887 

